Geochemistry of the Paleocene Clastic Rocks From the Southwestern Jianghan Basin, Central China Reveal Their Provenance, Tectonic Setting, and Palaeoenvironment

This paper intends to learn about the provenance, tectonic setting and paleoenvironment of the Paleocene Shashi Formation in the southern Jianghan Basin by the bulk-rock geochemistry. The K2O/Al2O3 and SiO2/Al2O3 ratios indicate that the major proportion of samples are litharenite. The chondrite-normalized REE distribution pattern of the Shashi Formation’s mudstones are characterized by enriched LREE and flat HREE similar to those of UC with negative Eu anomalies. Combined with the geochemical element ratio discriminant diagram, such as Al2O3-TiO2, Zr-TiO2, La/Sc-Co/Th, and Hf-La/Th, so on, these samples were sourced from mixed felsic/basic rock. Moreover, the discriminant diagrams of K2O/Na2O-SiO2/Al2O3, La-Th-Sc, and Th-Co-Zr/10 suggest that the samples were formed under the tectonic settings of active continental margin and continental island arc. The values of CIA, CIW, PIA, ICV, Zr/Sc-Th/Sc, and ternary diagrams of A-(CN)-K and Al2O3-Zr-TiO2 indicate that weathering in the source area was weak and source rocks have not been reformed by depositional recirculation and hydraulic sorting. And the palaeoenvironmental indicators of C-value, Ni/Co, V/Cr, V/(V+Ni) and Sr/Cu, Ga/Rb indicate that the climate was cool and arid during the evaporite deposition period in the southern Jianghan Basin, and the water was in the condition of oxidation.

Heavy Mineral Assemblage Variation in Late Cenozoic Sediments from the Middle Yangtze River Basin: Insights into Basin Sediment Provenance and Evolution of the Three Gorges Valley

The Three Gorges valley is one of the two key capture points of the evolution of the Yangtze River, yet the formation of this valley—from the pre-Miocene to the late Pleistocene—remains uncertain. The Jianghan Basin, a late Mesozoic–Cenozoic basin located just downstream of the Three Gorges valley, is a crucial area for understanding the formation of the valley. In this study, we used heavy mineral assemblages to trace the provenance of Pliocene–Pleistocene sediments obtained from the 300-m-depth Zhoulao drillcore in the Jianghan Basin. Results show that heavy mineral concentrations, compositions, and species display a clear change at a depth of 110 m in the studied core, consistent with the change in values of magnetic indexes and trace-element geochemical indicators. The heavy mineral assemblage deposited below a depth of 110 m (lower section of the core) comprises zircon, epidote, leucoxene, rutile, anatase, pyrite, and titanite, whereas that deposited above 110 m (upper section) consists of hornblende, pyroxene, garnet, hematite-limonite, and magnetite. In addition, the heavy mineral assemblage of the upper section is similar to that of the modern surface fluvial sediments of the Yangtze, which indicates that materials of the upper core section of the Jianghan Basin were sourced from the upper Yangtze River Basin, west of the Three Gorges. After incision of the Three Gorges valley, sediments from the upper Yangtze were transported to the Jianghan Basin and deposited. Combining the results of this study with the known paleomagnetic chronology of the Jianghan Basin, we propose that the Three Gorges valley was incised at ca. 1.1 Ma.

Oil Content Prediction Method Based on the TOC and Porosity of Organic-Rich Shales from Wireline Logs: A Case Study of Lacustrine Intersalt Shale Plays in Qianjiang Sag, Jianghan Basin, China

Organic-rich shales in between salt rock layers distribute widely in Qianjiang Sag, Jianghan Basin, central China. Due to the complexity of matrix mineral components and their distribution and tight pore structure, Archie’s law cannot be used directly to calculate oil saturation in those shale oil reservoirs. A new oil content model for shale oil reservoirs was introduced. By analyzing the logging and core experimental data from Qianjiang Sag, Jianghan Oilfield, we built the relationship between kerogen and the different well logging porosities including nuclear magnetic resonance (NMR) porosity, neutron porosity, and density porosity. And we used the dual- V sh method to calculate the total organic carbon (TOC). After calculating the volume fraction of the solid organic matters and separating it from the TOC, we acquired the hydrocarbon fluid content in the formations. The calculated oil content results are coherent with the core experimental data, which indicates the efficiency of this model. This model is simple and can be quickly applied. However, this method also shows its weakness in calculation precision when the TOC is not calculated precisely or the quality of the porosity logs is low.

Sedimentology, geochronology, and provenance of the late Cenozoic “Yangtze Gravel”: Implications for Lower Yangtze River reorganization and tectonic evolution in southeast China

The evolution of the Yangtze River, the longest river in Asia, provides a spectacular example for understanding the Cenozoic interaction between tectonics, climate, and surficial processes. The oldest Lower Yangtze deposits, characterized by ∼100-m-thick sequences of unconsolidated conglomerate, sandstone, and siltstone, referred to as “Yangtze Gravel,” have been recently dated >23 Ma, indicating a pre-Miocene establishment of a through-going river. However, the link between river integration and tectonic evolution has never been established due to the limited study of these sediments. Here, we report sedimentology, geochronology, and provenance of the Yangtze Gravel based on 17 stratigraphic sections exposed along the Lower Yangtze River. Our new chronostratigraphic results, including 40Ar/39Ar ages from the overlying basalt and fossil-based stratigraphic correlation, suggest an early-middle Miocene age for these sediments. Detailed analysis of lithofacies reveals several sequences of coarse-grained channel-belt deposits (channel fills and bars), indicating braided alluvial deposition across the Jianghan Basin, North Jiangsu-South Yellow Sea Basin, and East China Sea Shelf Basin. This ancient Lower Yangtze River is further characterized by petrography and detrital zircon U-Pb geochronology results which show similar provenance and erosion pattern as the present-day Yangtze River. However, the ancient river in early-middle Miocene is an alluvial, bedload-dominated braided river with higher stream power and a more prolonged course flowing into the East China Sea Shelf Basin. These differences between ancient and modern Lower Yangtze River reflect varied climate and paleogeography in southeast China during the late Cenozoic. Compared with the Paleogene red-colored, halite-bearing, Ephedripite pollen-dominated, lacustrine deposits in Jianghan Basin and North Jiangsu-South Yellow Sea Basin, the deposition of yellow to green-colored, coarse-grained, arboreal pollen, and wood-dominated Yangtze Gravel indicates a drainage reorganization from hydrologically closed lakes to a through-going river system during late Oligocene to early Miocene. During Paleogene, rift basins were filled by alluvial and fluvial-lacustrine deposition with restricted flow distance and local sources. From late Oligocene to early-middle Miocene, the post-rift subsidence opens a path for the ancient Lower Yangtze River connecting the Jianghan Basin, North Jiangsu-South Yellow Sea Basin, and East China Sea Shelf Basin. We attribute the drainage reorganization of the Lower Yangtze River to be a surficial response to Cenozoic tectonics, particularly the western Pacific subduction, in southeast China. The deposition of the widespread, coarse-grained Yangtze Gravel is probably due to the combined effects of catchment expansion and strong monsoonal climate in East Asia.